This invention relates to optical communications and, more particularly, to an optical multiplexing device used in an optical communications system.
In an optical communications system, information is encoded onto a light signal. The light signal is transmitted from one point to another, as for example by free-space light beams or optical fibers. At the receiving end, the information is read from the light signal.
An important advantage of optical communications is that a number of different light signals of different wavelengths may be mixed together (multiplexed) into a single light beam in a technique known as wavelength division multiplexing (WDM). Each light signal of a different wavelength, or channel, has information encoded onto it prior to the mixing of the channels. At the receiving end, the channels are separated, or demultiplexed, according to their wavelengths. The information on each channel is read from the demultiplexed light of that wavelength. A single multiplexed light beam may therefore carry many times the information that may be transmitted by a non-multiplexed light beam.
The multiplexing and/or demultiplexing may be accomplished using a series of light bandpass filters. Each filter is formed as a substrate and a multilayer dielectric light-transmissive optical stack deposited upon the substrate. The bandpass filter transmits only light of a specific wavelength. When a multichannel beam is incident upon the filter, the light channel associated with the bandpass range is transmitted through the filter to a light receiver behind the filter. The beam with the remaining channels is reflected to another filter, where the next channel is extracted from the beam in a similar fashion, and so on until all of the channels of information carried by the light beam have been separated for further processing.
As the number of channels transmitted on a light beam increases, the number of multiplexing or demultiplexing bandpass filters increases. The light beam that reaches each multiplexing or demultiplexing bandpass filter is reflected from the prior bandpass filter, so that any energy losses from the light beam due to angular misorientation or tolerances in the bandpass filters are multiplicative. For example, if there is a 2 percent loss of beam energy due to angular misorientation at each bandpass filter, the light energy of each channel reaching the next bandpass filter is 98 percent of the energy reaching the prior filter. If there are a large number of wavelength channels and corresponding bandpass filters, the reduction in light energy is significant. Additionally, existing multiplexers and demultiplexers of this type occupy a large area due to the reflective angular relationships.
An approach is needed to gain the advantages of the multiplexing and/or demultiplexing approach using filters, while avoiding the losses that are associated with the present approach. The present invention fulfills this need, and further provides related advantages.
The present invention provides an apparatus and method for the multiplexing and demultiplexing of light signals of different wavelengths transmitted in a common light beam. The present approach avoids multiplicative angular errors in a series of light-extraction modules. The angle of incidence of the light beam to each of the filters may be adjusted without concern for adversely affecting the downstream performance. The present approach also permits the plan-view footprint of the multiplexing device to be more generally linear rather than more generally rectangular.
In accordance with the invention, an optical multiplexing device comprises a first light band-reflect filter that receives an incident beam, reflects a first reflected beam of a first light wavelength band and transmits a first transmitted beam comprising light wavelengths other than the first light wavelength band, and a first receiver that receives the first reflected beam from the first light band-reflect filter. There is additionally a second light band-reflect filter that receives the first transmitted beam from the first light band-reflect filter, reflects a second reflected beam of a second light wavelength band and transmits a second transmitted beam comprising light wavelengths other than the second light wavelength band, and a second receiver that receives the second reflected light wavelength band from the second light band-reflect filter. There may be a light transmitter of the incident beam.
The present approach uses band-reflect filters that reflect the wavelength that is being extracted at each filter, rather than bandpass filters as in conventional optical multiplexing devices. The remainder of the beam is transmitted straight through the filter, so that the angle of the filter to the beam does not affect the transmitted beam. Instead, the angle of the filter to the beam is selected to achieve optimal discrimination of the wavelength that is being extracted by that filter, because the wavelength that is reflected is usually dependent upon the angle of incidence of the light beam to the filter. The first light band-reflect filter and the second light band-reflect filter may be of different constructions, with each filter construction optimized for the wavelength of light being reflected. They may instead be of same construction, and the wavelength selectivity is achieved by making the first angle of incidence of the incident beam upon the first light band-reflect filter different from the second angle of incidence of the first transmitted beam upon the second light band-reflect filter.
The optical multiplexing device may include a first alternate receiver, and a first switch mirror that controllably directs the first reflected beam to the first alternate receiver instead of the first receiver. The optical multiplexing device may further includes a second alternate receiver, and a second switch mirror that controllably directs the second reflected beam to the second alternate receiver instead of the second receiver.
The first light band-reflect filter may be structured to reflect the first reflected beam of the first light wavelength band, and transmits light of all other wavelengths. The second light band-reflect filter may be structured to reflect the second reflected beam of the second light wavelength band, and transmit light of all other wavelengths.
The first light band-reflect filter may instead be an edge filter. In a first embodiment, the first light band-reflect filter comprises a first high-pass edge filter having a first high-pass edge wavelength at an upper end of the first light wavelength band, and the second light band-reflect filter comprises a second high-pass edge filter having a second high-pass edge wavelength at an upper end of the second light wavelength band. In this first embodiment the second light wavelength band is at a higher wavelength than the first light wavelength band, and the light wavelength bands are extracted sequentially from lowest wavelength to highest wavelength. In a second embodiment, the first light band-reflect filter comprises a first low-pass edge filter having a first low-pass edge wavelength at a lower end of the first light wavelength band, and the second light band-reflect filter comprises a second low-pass edge filter having a second low-pass edge wavelength at a lower end of the second light wavelength band. In this second embodiment, the second light wavelength band is at a lower wavelength than the first light wavelength band, and the light wavelength bands are extracted sequentially from highest wavelength to lowest wavelength.
The optical multiplexing device is preferably used as a demultiplexer, but it may be used instead as a multiplexer based upon the reciprocal nature of the beam paths and functionality of the components. In that case, the first receiver and the second receiver are the same common receiver, and the second light band-reflect filter combines the second reflected beam and the second transmitted beam to form a mixed beam that is received by the common receiver.
Stated alternative, an optical multiplexing device comprises at least one light-extraction module intercepting a light beam. Each light-extraction module comprises a light band-reflect filter that receives the light beam, reflects a reflected beam of an extracted light wavelength band, and transmits the light beam except for the extracted light wavelength band, and a receiver that receives the reflected beam from the light band-reflect filter. There are preferably at least two of the light extraction modules, and preferably a plurality of the light extraction modules, with each light-extraction module having a different extracted light wavelength band. This approach may utilize the various features and embodiment discussed above.
By substituting band-reflect filters for bandpass filters, the present approach avoids multiplicative angular errors and allows the filters to be angle-tuned as necessary for optimal reflection and extraction of a selected wavelength of light. The multiplexing device may be made with a more-linear configuration that saves space in some applications.
The apparatus is more robust in conditions of vibration, stress, thermal excursions, and the like than are conventional multiplexing devices, because it is less sensitive to angular changes in the filter orientations. Thus, for example, if one of the extraction filters becomes misoriented due to vibration or the like, in the present approach only the information on the wavelength or channel being extracted at that extraction filter will be lost. The transmitted beam with the other wavelengths will be unaffected. In a conventional demultiplexer, if one of the filters becomes angularly misoriented by too large an amount, it will be impossible to extract any information from the subsequent beam that contains the other wavelengths.